Field of the Invention
The present invention relates to processes for the deposition of coatings on substrates, to apparatus for carrying out such processes and to products manufactured using such processes. The invention has particular, but not necessarily exclusive, application to modifications of known cold gas dynamic spraying processes.
Related Art
Cold gas dynamic spraying (typically known as “cold spraying” and referred to as such in the remainder of this specification) is a known process involving the entrainment of powder particles of a first material in a fast flowing stream of gas (typically a non-oxidising gas) and allowing the particles to impinge on a substrate formed of a second material. In this specification, the flowing powder particles are referred to as a “powder beam”. Under suitable conditions, the particles from the powder beam adhere to the substrate to form a coating layer on the substrate. The particles adhere to the substrate through plastic deformation and bonding. It is to be noted that neither the particles nor the substrate melt in this process (although some nanoscale local melting may be allowed). This is advantageous for many materials, since macroscopic melting can deleteriously affect the materials properties of the coating and/or the substrate.
In US 2006/0133947, a cold spraying process is modified by the use of a laser to provide heating. The intention in US 2006/0133947 is to improve the density of the coating. The powder particles have a diameter in the range 5-50 μm. Larger particle sizes typically cannot be accelerated to suitable speeds. Smaller particle sizes, whilst able to be accelerated to high speeds, tend to be swept away from the surface of the substrate due to a bow shock layer above the surface. The particles are accelerated to speeds in the range 850-1200 m/s. The preferred carrier gas in US 2006/0133947 is helium, because it allows the highest speeds to be obtained of any suitable gas. A laser is used to provide heating to increase the density of the coating after deposition. The laser may be moved with respect to the substrate, behind the powder beam applied to the substrate, in order to provide in situ heat treatment to the coating soon after the coating is deposited. There is no disclosure in US 2006/0133947 of heating the substrate directly using the laser.
GB-A-2439934 discloses a cold spray process in which it is intended to heat the powder particles in the gas stream. This is said to be achieved by both heating the gas and by directing a laser along the powder beam. The present inventors consider that it is doubtful that the powder particles are heated to a significant extent using the laser, considering the very short typical time of flight of the powder particles in view of the very high speeds required for cold spraying. However, GB-A-2439934 also discloses that the laser heats the substrate at the point at which the powder particles impact the substrate, giving rise to the effect of improved bonding between the powder particles and the substrate. There is no discussion in GB-A-2439934 of the power distribution of the laser beam.
US 2010/0068410 discloses a cold spray process in which a laser is directed to coincide with the point of impact of the particle beam, to provide local heating at that point. The stated aim of US 2010/0068410 is to manage the energy of the particles, so that they arrive at the substrate with just enough energy to adhere to the substrate and are then heated by the laser to fuse with the substrate. US 2010/0068410 also suggests that heating of the gas stream may be advantageous, in order to reduce the power requirements of the laser.
Christoulis et al (2010) [D. K. Christoulis, S. Guetta, E. Irissou, V. Guipont, M. H. Berger, M. Jeandin, J.-G. Legoux, C. Moreau, S. Costil, M. Boustie, Y. Ichikawa and K. Ogawa “Cold-Spraying Coupled to Nano-Pulsed Nd-YaG Laser Surface Pre-treatment” Journal of Thermal Spray Technology, Volume 19, Number 5, 1062-1073, 2010] disclose work on cold spraying of Al powder onto Al substrates. The carrier gas was nitrogen and the inlet gas temperature was 350° C. The Al substrates were subjected to laser ablation treatment using two Q-switched Nd-YAG lasers operating at a wavelength of 1.064 μm with an average power output of 40 W each (270 mJ per pulse with an adjustable frequency up to 150 Hz) and a pulse duration of about 10 ns. The laser beam was directed to the substrate so that the laser beam passed over the substrate milliseconds prior to the cold spray jet of particles.
Kulmala and Vuoristo (2008) [M. Kulmala and P. Vuoristo “Influence of process conditions in laser-assisted low-pressure cold spraying” Surface and Coatings Technology, Volume 202, Issue 18, 15 Jun. 2008, Pages 4503-4508] disclose cold spraying processes in which copper and nickel powders were sprayed with additions of alumina powder onto carbon steel substrates. The carrier gas was air, heated to 445° C. for copper and 650° C. for nickel. A 6 kW continuous wave laser was directed to heat the location at which the powder beam reached the substrate surface. The process described by this document is a low pressure process (gas pressure 6 bar). Incorporation of ceramic particles (alumina powder) into the powder beam assists in compaction of the deposited layer by mechanical means. The use of air as the carrier gas tends to cause at least partial oxidation of particles in the powder beam, further reducing the quality of the deposited layer.